5 Answers
5

I use polarizer so much that I never take it off of my main lens. The good thing about it that depending on its rotation you can adjust how strongly it affects you picture from maximum to virtually nothing.

Let's start with what the polarizer physically does:

it reduces (sometimes completely) the glare from non-metallic objects.

How does it translate to photographic situations?

enhances the blue of the sky and pulls out the detail on clouds

enhances the green of leaves and generally colors of plants and their flowers

makes the water and glass transparent

reduces the shining of human skin and gives it a soft appearance

it can remove the rainbow (which I imagine is rarely desired)

it enhances the overall contrast of the picture, creating very dramatic result in extreme cases

Examples of the same subject non-polarized (left) and polarized (right):

When does it work best?

when the camera is facing on an angle 90° to the sun and the direction marker is positioned towards sun (hope it's clear - I'm not quite happy with the wording here, will fix it when I come up with something better)

What to be careful about?

when overused polarizer can give photos a very unnatural appearance (which I personally enjoy)

+1 very good Answer! Which polarizer filter would you suggest for a Nikon 18-55mm lens? I have seen there are many, but I do not know which could be considered an "all-around" filter fitting the most situations.
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LucaJan 12 '12 at 10:35

"when the camera is facing on an angle 90°", does it mean that if the sun is directly above your head, then it will work best when you shoot to the horizon? So far I have not been successful in achieving pictures like yours, seems like the picture does not change at all no matter how I rotate the filter.
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rcsDec 23 '13 at 0:10

Apart from what has already been mentioned (decreasing reflections from water and glass surfaces, bringing out that tourist-brochure blue skies with bright white clouds) it is worth to mention that it also decreases reflections from leaves on plants. Using a polarizer when shooting landscapes with a lot of vegetation helps bringing out a deeper, more lush green tone from the vegetation.

A polarizing filter removes secondary light, i.e. light that has bounced on a surface and become polarized.

This happens when light bounces off of things like a water surface, glass window, and even in the atmosphere. (Light that bounces off a metal surface doesn't get polarized though.)

If you filter out the light that bounces in the athmospehere, you will get a darker blue sky in your images. When you filter out polarized light in the sky, you get the highest effect if you point the camera 90 degrees away from the sun, and almost no effect if you point the camera towards the sun or away from it.

A circular polarizing filter is used in a SLR camera. As there is a mirror in the camera, the light has to be de-polarized after it has been filtered, or you would not see anything at all in the viewfinder when the filter is turned at certain angles.

Polarizers do block light, as they are designed to filter out light coming from certain directions. Polarizers are useful for a variety of reasons:

Mitigating highlights on water, rocks, shiny plant leaves, etc.

Enhancing the sky, particularly blue sky, and improving the contrast balance of a scene

Bringing out the detail in clouds by reducing their glare

A polarizing filter, particularly circular polarizers, are a great tool to balance out and enhance a scene. They do reduce the rate at which light flows into your lens, so a slight exposure adjustment is usually necessary to compensate. They are generally worth it, if you have one, and can help improve your shots.